To date, most transistors used in power electronic applications have typically been fabricated with silicon (Si) semiconductor materials. Common transistor devices for power applications include Si CoolMOS, Si Power MOSFETs, and Si Insulated Gate Bipolar Transistors (IGBTs). While Si power devices are inexpensive, they suffer from a number of disadvantages, including relatively low switching speeds and high levels of electrical noise. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-Nitride or III-N semiconductor devices, such as gallium nitride (GaN) devices, are now emerging as attractive candidates to carry large currents, support high voltages, and to provide very low on-resistance and fast switching times.
FIGS. 1A and 1B illustrate a plan view and a cross-sectional view, respectively, of a III-N transistor of the prior art having source electrode 14, drain electrode 15, gate electrode 13, and access regions 23 and 24. As used herein, the “access regions” of a transistor refer to the two regions between the source and gate electrodes, and between the gate and drain electrodes of the transistor, i.e., regions 23 and 24, respectively, in FIGS. 1A and 1B. Region 23, the access region on the source side of the gate, is typically referred to as the source access region, and region 24, the access region on the drain side of the gate, is typically referred to as the drain access region. As used herein, the “gate region” 31 of a transistor refers to the portion of the transistor between the two access regions 23 and 24 in FIGS. 1A and 1B.
While numerous III-N transistors and diodes have been demonstrated, improvements in device reliability are still necessary in order to enable large scale manufacturing and more widespread adoption of these devices. In particular, in III-N devices configured to support high voltages and/or large currents, large electric fields present in the device during operation can lead to deleterious effects such as threshold voltage shifts and breakdown, as well as other effects that degrade device performance or otherwise render the device inoperable. Device structures that reduce such degradation and improve device reliability are therefore desirable.